1. Field of the Invention
The present invention relates to the production of reticles for extreme ultraviolet lithography, and more specifically, it relates to systems and methods for directly writing patterns onto the reflective multilayer coating of an extreme ultraviolet lithography reticle.
2. Description of Related Art
The standard reflective reticle blank for extreme ultraviolet lithography (EUVL) consists of a thick substrate coated with a reflective multilayer film. The reflective multilayer coating may consist of any of several different material combinations. The industry standard is 40 layer pairs of molybdenum and silicon. Each layer pair has a thickness of about 7 nm. To fabricate the EUVL reticle, a buffer-layer film of thickness 50-100 nm plus an absorber film of thickness between 50 and 150 nm is deposited on the multilayer reflective coating and subsequently patterned. The buffer-layer typically consists of SiO2, and is used to protect the multilayer during patterning and serves as a sacrificial layer for the repair of absorber defects. Absorber materials such as Al, Cr or TiN produce a binary modulation of the reflected field according to the spatial pattern to generate the desired lithographic image. There are significant costs and issues associated with this process. For example, the buffer-layer film needs to be of sufficient thickness to protect the multilayer, but as the (transparent) buffer-layer thickness increases the absorber is placed higher above the multilayer surface. For extreme ultraviolet (EUV) light incident on the multilayer film at angles away from normal incidence, this results in reflected EUV tight escaping from the multilayer where it typically would be caught by the absorber.
Reticles for EUV lithography other than the one discussed above have also been proposed. One design uses a patterned multilayer film, where the non-reflecting substrate effectively acts as the absorber. There are disadvantages with this approach, including the sharpness of the absorber edges that are produced. Another EUVL reticle proposal is to use a focused beam to pattern a multilayer-coated reticle by destroying the reflectance of the multilayer in the areas that are exposed to the beam. The main disadvantage of this approach is the difficulty in confining the affected region to a small area considering the significant damage that must be done to make the multilayer non-reflecting.
U.S. Pat. No. 5,521,031, titled “Pattern Delineating Apparatus for use in the EUV spectrum” proposes a variant on the standard EUVL reticle. The patent incorporates the basic principles of an attenuated phase shift reticle in a reflecting structure. This allows a small amount of EUV light to be reflected from absorber layers, where the EUV light is phase shifted by 180° relative to the reflective multilayer. This allows for an increase in the sharpness of the EUV light intensity modulation at the wafer plane, and hence greater resolution.
It is desirable to eliminate the absorber material from an EUVL reticle by introducing a direct modulation in the complex-valued reflectance of an EUVL reticle thin film multilayer.